Creating YAML Mechanism Files from Scratch#

Virtually every Cantera simulation involves one or more phases of matter. Depending on the calculation being performed, it may be necessary to evaluate thermodynamic properties, transport properties, and/or reaction rates for the phase(s) present. Before the properties can be evaluated, each phase must be defined, meaning that the models to use to compute its properties and reaction rates must be specified, along with any parameters the models require.

Because the amount of data required can be quite large, this data is imported from a YAML file that can be read by the application, so that a given phase model can be re-used for other simulations. This guide describes how to write such files to define phases and interfaces for use in Cantera simulations.

See also

See the Introduction to YAML Input Files for an introduction to the YAML syntax used by Cantera and a description of how dimensional values are handled.

Phases#

For each phase that appears in a problem, a corresponding entry should be present in the phases section of the input file. The phase entry specifies the elements and species present in that phase, and the models to be used for computing thermodynamic, kinetic, and transport properties.

Naming the Phase#

The name entry is a string that identifies the phase. It must be unique within the file among all phase definitions of any type. Phases are referenced by name when importing them. The name is also used to identify the phase within multiphase mixtures or at phase boundaries.

Setting the Thermodynamic Model#

The thermodynamic model used to represent a phase is specified in the thermo field. A complete list of supported models can be found in the YAML Input File Reference. Some thermodynamic models use additional fields in the phase entry, which are described in the linked documentation.

Declaring the Elements#

In most cases, it is not necessary to specify the elements present in a phase. If no elements field is present, elements will be added automatically using the definitions of the standard chemical elements based on the composition of the species present in the phase.

If non-standard elements such as isotopes need to be represented, or the ordering of elements within the phase is important, the elements in the phase may be declared in the optional elements entry.

If all of the elements to be added are either standard chemical elements or defined in the elements section of the current input file, the elements can be specified as a list of element symbols. For example:

phases:
- name: my-mechanism
  elements: [H, C, O, Ar]
  ...

To add elements from other top-level sections, from a different file, or from multiple such sources, a list of single-key mappings can be used where the key of each mapping specifies the source and the value is a list of element names. The keys can be:

  • The name of a section within the current file.

  • The name of an input file and a section in that file, separated by a slash, for example myfile.yaml/my_elements. If a relative path is specified, the directory containing the current file is searched first, followed by the Cantera data path.

  • The name default to reference the standard chemical elements.

Example:

my-isotopes:
- name: O18
  atomic-weight: 17.9991603
phases:
- name: my-phase
  elements:
  - default: [C, H, Ar]
  - my-isotopes: [O18]
  - myelements.yaml/uranium: [U235, U238]
  species: ...
  ...

The order of the elements specified in the input file determines the order of the elements in the phase when it is imported by Cantera.

Declaring the Species#

If the species present in the phase corresponds to those species defined in the species section of the input file, the species field may be omitted, and those species will be added to the phase automatically. As a more explicit alternative, the species field may be set to the string all.

To include specific species from the species section of the input file, the species entry can be a list of species names from that section. For example:

phases:
- name: my-phase
  species: [H2, O2, H2O]
  ...

If species are defined in multiple input file sections, the species entry can be a list of single-key mappings, where the key of each mapping specifies the source and the value is either the string all or a list of species names. Each key can be either the name of a section within the current input file or the name of a different file and a section in that file, separated by a slash. If a relative path is specified, the directory containing the current file is searched first, followed by the Cantera data path. Example:

phases:
- name: my-phase
  species:
  - species: [O2, N2]
  - more_species: all
  - subdir/myfile.yaml/species: [NO2, N2O]
  ...

The order of species specified in the input file determines the order of the species in the phase when it is imported by Cantera.

Species containing elements that are not declared within the phase may be skipped by setting the skip-undeclared-elements field to true. For example, to add all species from the species section that contain only hydrogen or oxygen, the phase definition could contain:

phases:
- name: hydrogen-and-oxygen
  elements: [H, O]
  species: all
  skip-undeclared-elements: true
  ...

Setting the Kinetics Model#

The kinetics model to be used, if any, is specified in the kinetics field. Supported kinetics models are bulk, surface, and edge, depending on the dimensionality of the phase. If omitted, no kinetics model will be used. For additional details, see the list of supported models in the YAML Input File Reference.

Declaring the Reactions#

If a kinetics model has been specified, reactions may be added to the phase. By default, all reactions from the reactions section of the input file will be added. Equivalently, the reactions entry may be specified as the string all.

To disable automatic addition of reactions from the reactions section, the reactions entry may be set to none. This may be useful if reactions will be added programmatically after the phase is constructed. The reactions field must be set to none if a kinetics model has been specified but there is no reactions section in the input file.

To include only those reactions from the reactions section where all of the species involved are declared as being in the phase, the reactions entry can be set to the string declared-species.

To include reactions from multiple sections or other files, the reactions entry can be given as a list of section names, for example:

phases:
- name: my-phase
  ...
  reactions:
  - OH_submechanism
  - otherfile.yaml/C1-reactions
  - otherfile.yaml/C2-reactions
  ...

To include reactions from multiple sections or other files while only including reactions involving declared species, a list of single-key mappings can be used, where the key is the section name (or file and section name) and the value is either the string all or the string declared-species. For example:

phases:
- name: my-phase
  ...
  reactions:
  - OH_submechanism: all
  - otherfile.yaml/C1-reactions: all
  - otherfile.yaml/C2-reactions: declared-species
  ...

To permit reactions containing third-body efficiencies for species not present in the phase, the additional field skip-undeclared-third-bodies may be added to the phase entry with the value true.

Setting the Transport Model#

To enable transport property calculation, the transport model to be used can be specified in the transport field. A complete list of supported models can be found in the YAML Input File Reference. For most transport models, additional parameters need to be specified within each species definition.

Declaring Adjacent Phases#

For interface phases (surfaces and edges), the names of phases adjacent to the interface can be specified, in which case these additional phases can be automatically constructed when creating the interface phase. This behavior is useful when the interface has reactions that include species from one of these adjacent phases, since those phases must be known when adding such reactions to the interface.

If the definitions of the adjacent phases are contained in the phases section of the same input file as the interface, they can be specified as a list of names:

phases:
- name: my-surface-phase
  ...
  adjacent: [gas, bulk]
  ...
- name: gas
  ...
- name: bulk
  ...

Alternatively, if the adjacent phase definitions are in other sections or other input files, they can be specified as a list of single-key mappings where the key is the section name (or file and section name) and the value is the phase name:

phases:
- name: my-surface-phase
  ...
  adjacent:
  - {my-other-phases: gas}
  # a phase defined in the 'phases' section of a different YAML file
  - {path/to/other-file.yaml/phases: bulk}
  ...
my-other-phases:
- name: gas
  ...

Since an interface kinetics mechanism is defined for the lowest-dimensional phase involved in the mechanism, only higher-dimensional adjacent phases should be specified. For example, when defining a surface, adjacent bulk phases may be specified, but adjacent edges must not.

Setting the Initial State#

The initial state of a phase can be set using two properties to set the thermodynamic state, plus the composition. This state is specified as a mapping in the state field of phase entry.

The thermodynamic state can be set by specifying two properties, such as temperature (T) and pressure (P) or internal energy (U) and density (D). The full list of property names and valid combinations can be found in the YAML Input File Reference. In addition the composition can be set by providing a mapping that gives the mass fractions (X), mole fractions (Y), coverages (for surface phases), or molalities (M, for certain solution models). Where necessary, the values will be automatically normalized.

Some examples of setting the state:

phases:
- name: my-phase
  ...
  state:
    T: 300 K
    P: 101325 Pa
    X: {O2: 1.0, N2: 3.76}
- name: my-other-phase
  ...
  state:
    density: 100 kg/m^3
    T: 298
    Y:
      CH4: 0.2
      C3H8: 0.1
      CO2: 0.7

For pure fluid phases, the temperature, pressure, and vapor fraction may all be specified if and only if they define a consistent state.

Examples#

The following input file defines two equivalent gas phases including all reactions and species defined in the input file. The species and reaction data is not shown for clarity. In the second case, the phase definition is simplified by having the elements added based on the species definitions, taking the species definitions from the default species section, and reactions from the default reactions section.

phases:
- name: gas1
  thermo: ideal-gas
  elements: [O, H, N, Ar]
  species: [H2, H, O, O2, OH, H2O, HO2, H2O2, N2, AR]
  kinetics: gas
  reactions: all
  transport: mixture-averaged
  state:
    T: 300.0
    P: 1.01325e+05
- name: gas2
  thermo: ideal-gas
  kinetics: gas
  transport: mixture-averaged
  state: {T: 300.0, 1 atm}

species:
- H2: ...
- H: ...
...
- AR: ...

reactions:
...

An input file defining an interface and its adjacent bulk phases, with full species data not shown for clarity:

phases:
- name: graphite
  thermo: lattice
  species:
  - graphite-species: all
  state: {T: 300, P: 101325, X: {C6: 1.0, LiC6: 1e-5}}
  density: 2.26 g/cm^3

- name: electrolyte
  thermo: lattice
  species: [{electrolyte-species: all}]
  density: 1208.2 kg/m^3
  state:
    T: 300
    P: 101325
    X: {Li+(e): 0.08, PF6-(e): 0.08, EC(e): 0.28, EMC(e): 0.56}

- name: anode-surface
  thermo: ideal-surface
  adjacent: [graphite, electrolyte]
  kinetics: surface
  reactions: [graphite-anode-reactions]
  species: [{anode-species: all}]
  site-density: 1.0 mol/cm^2
  state: {T: 300, P: 101325}

graphite-species:
- name: C6
  ...
- name: LiC6
  ...

electrolyte-species:
- name: Li+(e)
  ...
- name: PF6-(e)
  ...
- name: EC(e)
  ...
- name: EMC(e)
  ...

anode-species:
- name: (int)
  ...

graphite-anode-reactions:
- equation: LiC6 <=> Li+(e) + C6
  rate-constant: [5.74, 0.0, 0.0]
  beta: 0.4

Species#

For each species declared as part of a phase description, a species definition is required to describe the composition, thermodynamic, and transport parameters of that species.

The default location for species entries is in the species section of the input file. Species defined in this section will automatically be considered for addition to phases defined in the same file. Species can be defined in other sections of the input file or in other input files, and these species definitions can be used in phase definitions by explicitly referencing the section name.

Species Name#

The name of a species is given in the name field of a species entry. Names may include almost all printable characters, with the exception of spaces. The use of some characters such as [, ], and , may require that species names be enclosed in quotes when written in YAML. Some valid species names given in a YAML list include:

[CH4, methane, argon_2+, "C[CH2]", CH2(singlet), "H2O,l"]

Elemental Composition#

The elemental composition of a species is specified as a mapping in the composition entry.

For gaseous species, the elemental composition is well-defined, since the species represent distinct molecules. For species in solid or liquid solutions, or on surfaces, there may be several possible ways of defining the species. For example, an aqueous species might be defined with or without including the water molecules in the solvation cage surrounding it.

For surface species, it is possible for the composition mapping to be empty, in which case the species is composed of nothing, and represents an empty surface site. This can also be done to represent vacancies in solids. A charged vacancy can be defined to be composed solely of electrons.

The special pseudo-element E is used in representing charged species, where it specifies the net number of electrons compared to the number needed to form a neutral species. That is, negatively charged ions will have E > 0, while positively charged ions will have E < 0.

The number of atoms of an element must be non-negative, except for electrons.

Examples:

composition: {C: 1, O: 2}  # carbon dioxide
composition: {Ar: 1, E: -2}  # Ar++
composition: {Y: 1, Ba: 2, Cu: 3, O: 6.5}  # stoichiometric YBCO
composition: {}  # A surface species representing an empty site

Thermodynamic Properties#

In addition to the thermodynamic model used at the phase level for computing properties, parameterizations are usually required for the enthalpy, entropy, and specific heat capacities of individual species under standard conditions. These parameterizations are provided in the thermo field of each species entry, with the type of parameterization specified by the model field. A complete list of parameterizations and their model-specific fields can be found in the YAML Input File Reference.

An example thermo field using the 7-coefficient NASA polynomials in two temperature regions:

thermo:
  model: NASA7
  temperature-ranges: [200.0, 1000.0, 3500.0]
  data:
  - [5.14987613, -0.0136709788, 4.91800599e-05, -4.84743026e-08, 1.66693956e-11,
    -1.02466476e+04, -4.64130376]
  - [0.074851495, 0.0133909467, -5.73285809e-06, 1.22292535e-09, -1.0181523e-13,
    -9468.34459, 18.437318]

Species Equation of State#

For some phase thermodynamic models, additional equation of state parameterizations are needed for each species. This information is provided in the equation-of-state field of each species entry, with the type of parameterization used specified by the model field of the equation-of-state field. A complete list of equation of state parameterizations and their model-specific fields can be found in the YAML Input File Reference.

Species Transport Coefficients#

Transport-related parameters for each species are needed in order to calculate transport properties of a phase. These parameters are provided in the transport field of each species entry, with the type of the parameterization used specified by the model field of the transport field. The only model type specifically handled is gas. The parameters used depend on the transport model specified at the phase level. The full set of possible parameters is described in the API documentation.

An example of a transport entry for a gas-phase species:

transport:
  model: gas
  geometry: linear
  well-depth: 107.4
  diameter: 3.458
  polarizability: 1.6
  rotational-relaxation: 3.8

Reactions#

Cantera supports a number of different types of reactions, including several types of homogeneous reactions, surface reactions, and electrochemical reactions. The reaction entries for all reaction types some common features. These general fields of a reaction entry are described first, followed by fields used for specific reaction types.

The Reaction Equation#

The reaction equation, specified in the equation field of the reaction entry, determines the reactant and product stoichiometry. All tokens (species names, stoichiometric coefficients, +, and <=>) in the reaction equation must be separated with spaces. Some examples of correctly and incorrectly formatted reaction equations are shown below:

- equation: 2 CH2 <=> CH + CH3  # OK
- equation: 2 CH2<=>CH + CH3  # error - spaces required around '<=>''
- equation: 2CH2 <=> CH + CH3  # error - space required between '2' and 'CH2'
- equation: CH2 + CH2 <=> CH + CH3  # OK
- equation: 2 CH2 <=> CH+CH3  # error - spaces required around '+'

Whether the reaction is reversible or not is determined by the form of the equality sign in the reaction equation. If either <=> or = is found, then the reaction is regarded as reversible, and the reverse rate will be computed based on the equilibrium constant. If, on the other hand, => is found, the reaction will be treated as irreversible.

Reaction type#

The type of the rate coefficient parameterization may be specified in the type field of the reaction entry. The fields for specific reaction types and additional parameters defining the rate constant for each of these reaction types are described in the YAML Input File Reference.

The default parameterization is elementary. Reactions involving surface species are automatically identified as interface reactions, reactions involving surface species with the type specified as Blowers-Masel are treated as interface-Blowers-Masel, and reactions involving charge transfer are automatically identified as electrochemical reactions.

Arrhenius Expressions#

Most reaction types in Cantera are parameterized by one or more modified Arrhenius expressions, such as

\[ A T^b e^{-E_a / RT} \]

where \(A\) is the pre-exponential factor, \(T\) is the temperature, \(b\) is the temperature exponent, \(E_a\) is the activation energy, and \(R\) is the gas constant. Rates in this form can be written as YAML mappings. For example:

{A: 1.0e13, b: 0, E: 7.3 kcal/mol}

The units of \(A\) can be specified explicitly if desired. If not specified, they will be determined based on the quantity, length, and time units specified in the governing units fields. Since the units of \(A\) depend on the reaction order, the units of each reactant concentration (dependent on phase type and dimensionality), and the units of the rate of progress (different for homogeneous and heterogeneous reactions), it is usually best not to specify units for \(A\), in which case they will be computed taking all of these factors into account.

Note

If \(b \ne 0\), then the term \(T^b\) should have units of \(\t{K}^b\), which would change the units of \(A\). This is not done, however, so the units associated with \(A\) are really the units for \(k_f\). One way to formally express this is to replace \(T^b\) by the non-dimensional quantity \([T/(1\;\t{K})]^b\).

The key E is used to specify \(E_a\).

Duplicate Reactions#

When a reaction is imported into a phase, it is checked to see that it is not a duplicate of another reaction already present in the phase, and normally an error results if a duplicate is found. But in some cases, it may be appropriate to include duplicate reactions, for example if a reaction can proceed through two distinctly different pathways, each with its own rate expression. Another case where duplicate reactions can be used is if it is desired to implement a reaction rate coefficient of the form:

\[ k_f(T) = \sum_{n=1}^{N} A_n T^{b_n} \exp(-E_n/RT) \]

While Cantera does not provide such a form for reaction rates, it can be implemented by defining \(N\) duplicate reactions, and assigning one rate coefficient in the sum to each reaction. By adding the field:

duplicate: true

to a reaction entry, then the reaction not only may have a duplicate, it must. Any reaction that specifies that it is a duplicate, but cannot be paired with another reaction in the phase that qualifies as its duplicate generates an error.

Negative Pre-exponential Factors#

If some of the terms in the above sum have negative \(A_n\), this scheme fails, since Cantera normally does not allow negative pre-exponential factors. But if there are duplicate reactions such that the total rate is positive, then the fact that negative \(A\) parameters are acceptable can be indicated by adding the field:

negative-A: true

Reaction Orders#

Explicit reaction orders different from the stoichiometric coefficients are sometimes used for non-elementary reactions. For example, consider the global reaction:

\[ \t{C_8H_{18} + 12.5 O_2 \rightarrow 8 CO_2 + 9 H_2O} \]

the forward rate constant might be given as [Westbrook and Dryer, 1981]:

\[ k_f = 4.6 \times 10^{11} [\t{C_8H_{18}}]^{0.25} [\t{O_2}]^{1.5} \exp\left(\frac{30.0\,\t{kcal/mol}}{RT}\right) \]

This reaction could be defined as:

- equation: C8H18 + 12.5 O2 => 8 CO2 + 9 H2O
  rate-constant: {A: 4.6e11, b: 0.0, Ea: 30.0 kcal/mol}
  orders: {C8H18: 0.25, O2: 1.5}

Special care is required in this case since the units of the pre-exponential factor depend on the sum of the reaction orders, which may not be an integer.

Note that you can change reaction orders only for irreversible reactions.

Negative Reaction Orders#

Normally, reaction orders are required to be positive. However, in some cases negative reaction orders provide better fits for experimental data. In these cases, the default behavior may be overridden by adding the negative-orders field to the reaction entry. For example:

- equation: C8H18 + 12.5 O2 => 8 CO2 + 9 H2O
  rate-constant: {A: 4.6e11, b: 0.0, Ea: 30.0 kcal/mol}
  orders: {C8H18: -0.25, O2: 1.75}
  negative-orders: true

Non-reactant Orders#

Some global reactions could have reactions orders for non-reactant species. In this case, the nonreactant-orders field must be added to the reaction entry:

- equation: C8H18 + 12.5 O2 => 8 CO2 + 9 H2O
  rate-constant: {A: 4.6e11, b: 0.0, Ea: 30.0 kcal/mol}
  orders: {C8H18: -0.25, CO: 0.15}
  negative-orders: true
  nonreactant-orders: true

Elements#

In Cantera, an element may refer to a chemical element or an isotope. Cantera provides built-in definitions for the chemical elements, including values for their atomic weights taken from IUPAC / CIAAW. These elements can be used by specifying the corresponding atomic symbols when specifying the composition of species. Explicit element definitions are usually only needed for isotopes.

In order to give a name to a particular isotope or a virtual element representing a surface site, a custom element entry can be used. The default location for element entries is the elements section of the input file. Elements defined in this section will automatically be considered for addition to phases defined in the same file. Elements can be defined in other sections of the input file if those sections are named explicitly in the elements field of the phase definition.

An element entry has the following fields:

  • symbol: The symbol to be used for the element, for example when specifying the composition of a species.

  • atomic-weight: The atomic weight of the element, in unified atomic mass units (dalton)

  • atomic-number: The atomic number of the element. Optional.

  • entropy298: The standard molar entropy of the element at 298.15 K. Optional.

An example elements section:

elements:
- symbol: C13
  atomic-weight: 13.003354826
  atomic-number: 6
- symbol: O-18
  atomic-weight: 17.9991603
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